I. Field of the Invention
The present invention relates to communications. More particularly, the present invention relates to a method and apparatus for transmitting and receiving variable rate packets of data with signals indicative of the data rate of those packets.
II. Description of the Related Art
The use of code division multiple access (CDMA) modulation techniques is one of several techniques for facilitating communications in which a large number of system users are present. Although other techniques such as time division multiple access (TDMA), frequency division multiple access (FDMA), and AM modulation schemes such as amplitude companded single sideband (ACSSB) are known, CDMA has significant advantages over these other techniques. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled xe2x80x9cSPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,xe2x80x9d and assigned to the assignee of the present invention and incorporated by reference herein. The use of CDMA techniques in a multiple access communication system is further disclosed in U.S. Pat. No. 5,103,459, entitled xe2x80x9cSYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM,xe2x80x9d assigned to the assignee of the present invention and incorporated by reference herein.
In the aforementioned U.S. Pat. No. 5,103,459 (the ""459 patent), the use of orthogonal Walsh codes to provide channelization to different subscriber stations is described. This allows a base station to transmit many separate channels to a plurality of users in the coverage area of the base station. In the ""459 patent, one of the orthogonal Walsh channels that was transmitted was a pilot channel that allowed for the coherent demodulation of the traffic channels transmitted on other orthogonal Walsh channels. A method for transmitting a CDMA signal from a mobile station which is capable of coherent demodulation is described in U.S. patent application Ser. No. 08/856,428, now abandoned, entitled xe2x80x9cREDUCED PEAK TO AVERAGE TRANSMIT POWER HIGH DATA RATE IN A CDMA WIRELESS COMMUNICATION SYSTEM,xe2x80x9d filed May 14, 1997, assigned to the assignee of the present invention and incorporated by reference herein. In U.S. patent application Ser. No. 08/856,428, the mobile station transmits a plurality of different channels wherein each of the channels is distinguished by use of a short Walsh sequence. In addition, U.S. patent application Ser. No. 08/856,428 describes a method of complex pseudonoise (PN) spreading that reduces peak to average ratio in the transmission of a QPSK modulated signal.
CDMA systems often employ a variable rate vocoder to encode data so that the data rate can be varied from one data frame to another. An exemplary embodiment of a variable rate vocoder is described in U.S. Pat. No. 5,414,796, entitled xe2x80x9cVARIABLE RATE VOCODER,xe2x80x9d assigned to the assignee of the present invention and incorporated by reference herein. The use of a variable rate communications channel reduces mutual interference by eliminating unnecessary transmissions when there is no useful speech to be transmitted.
Similarly, it is desirable for providing variable rate transmission of digital data in CDMA wireless communication systems. When there is a great deal of digital information to be transmitted and when minimizing delay is important, then data should be transmitted at high transmission rates. However, when there is less data to be transmitted or when minimizing delay is not as important, it is desirable to reduce the transmission rate of digital data in a wireless communication system, because transmission at rates lower than the maximum transmission rate can result in increased range, extended battery life and reduce interference to other users.
One technique for the receiver to determine the rate of a received data frame is described in U.S. Pat. No. 5,566,206, entitled xe2x80x9cMETHOD AND APPARATUS FOR DETERMINING DATA RATE OF TRANSMITTED VARIABLE RATE DATA IN A COMMUNICATIONS RECEIVER,xe2x80x9d assigned to the assignee of the present invention and incorporated by reference herein. Another technique is described in U.S. patent application Ser. No. 08/126,477, entitled xe2x80x9cMULTIRATE SERIAL VITERBI DECODER A FOR CODE DIVISION MULTIPLE ACCESS SYSTEM APPLICATIONS,xe2x80x9d filed Sep. 24, 1993, now U.S. Pat. No. 5,710,784, issued Jan. 20, 1998 to Kindred et al., assigned to the assignee of the present invention, and incorporated by reference herein. According to these techniques, each received data frame is decoded at each of the possible rates. Error metrics, which describe the quality of the decoded symbols for each frame decoded at each rate, are provided to a processor. The error metrics may include Cyclic Redundancy Check (CRC) results, Yamamoto Quality Metrics, and Symbol Error Rates. These error metrics are well-known in communications systems. The processor analyzes the error metrics and determines the most probable rate at which the incoming symbols were transmitted.
The present invention provides a novel and improved apparatus and method for transmitting and receiving variable rate data. In the first embodiment of the present invention, the data is spread using a long pseudonoise code generated by a linear feedback PN generator, the mask of which is selected in accordance with the transmission rate of the variable data and the specific user transmitting the data. Thus, by identifying at the receiver which mask will allow the received waveform to be correctly despread, the rate of the data can be determined. In the second embodiment of the present invention a preamble from a predetermined set of preambles is punctured into the outgoing pilot signal to provide rate indication information. In the third embodiment, the rate indication signal is encoded in accordance with a set of orthogonal functions which are part of the indication of the rate of the data packet.